The invention relates to imaging, and more particularly to laser-induced thermal imaging.
An image may be encoded into image data, which may be transmitted, stored, processed, or otherwise manipulated electronically. The image may be decoded and converted to hard copy by sending the image data to a printer. Laser thermal printers, with their high resolution capability, provide a popular mode for producing hard copy images from digital image data.
Laser thermal printers may be used with a variety of imaging media or xe2x80x9creceptors,xe2x80x9d including many kinds of film and paper. In a typical laser imager, a receptor is placed very close to a color-coated substrate or xe2x80x9cdonorxe2x80x9d sheet, and a plurality of laser beams are directed at the donor. Each laser may emit an infrared beam, and the colored coating, which may contain a colorant and/or an infrared-sensitive dye, heats when exposed to a beam of sufficient power. The resulting thermal energy induced by the lasers triggers the imaging process, causing colorant to transfer from the donor to the receptor.
The lasers are typically arranged in a linear array, with each laser in the array individually modulated by image data. The array may include any number of lasers, although an array of sixteen lasers is typical. Semiconductor or xe2x80x9cdiodexe2x80x9d lasers are commonly used in an array for reasons of cost, convenience and reliability. The lasers may, for example, emit infrared beams with wavelengths of approximately 830 nm. The width of the array, which is a function of the spacing of the lasers, is usually adjustable.
The image data that modulate the lasers represent the shape, size, color and density of the image. Image data are routinely stored electronically, and are provided to the array in the form of a plurality of signals, typically one signal for each laser. Although the lasers in the array strike only a small portion of the donor and receptor at any one time, the array can print large regions by scanning across the donor and receptor. As the array scans the donor and receptor, each laser in the array emits a beam in response to an image signal. In most cases, the laser array may make several successive parallel or helical passes to generate the complete image. Each pass of the array prints a strip or xe2x80x9cswathxe2x80x9d on the receptor. To avoid the appearance of white lines in the receptor, i.e., unprinted spaces between swaths, successive swaths may abut or overlap preceding swaths.
When a beam sufficiently heats the donor, a spot of colorant is transferred from the donor to the receptor. By modulating the duration for which a laser beam strikes an area on the donor, modulating a laser""s intensity and/or modulating the size of the beam, spots of colorant of different sizes may be formed, and thereby colors may appear darker or fainter in color. Often a region of the receptor is intended to receive no colorant from the donor, and when scanning that region, a laser emits a beam at a power level below that needed to transfer colorant.
The receptor may be scanned multiple times using donors of different colorants, creating a multicolor image by the superposition of multiple monochromatic images. By repeating scans with donors coated with cyan, yellow, magenta and black, for example, a multicolor image may be formed on the receptor. For high fidelity printing systems, additional colors such as green and orange may be provided.
The invention is directed to compensating for imaging aberrations, sometimes referred to as xe2x80x9cartifacts,xe2x80x9d that result from repeated passes by an imaging laser array. The invention is particularly useful in a thermal imaging system that makes use of a laser array. Ideally, an observer ought to be able to look at a printed image and see no indications that the image had been formed by repeated passes of a laser array. In some cases, however, unintended patterns, such as groups of lines or streaks, appear in the printed image. Such artifacts may be evident in halftone printing, when printing a single color and when overprinting multiple colors.
At least two factors contribute to these artifacts. One factor is the formation of xe2x80x9cswath lines,xe2x80x9d which may manifest at an edge of a laser swath in the form of heavier colorant depositions. Swath lines may result from the deposition of excess colorant along an edge of the swath, causing a heavier line of colorant than intended and causing the swath to have a non-uniform distribution. Swath lines may also result from a deficit of colorant. The end lasers in the array, i.e., the outermost lasers in the array, have one neighbor laser instead of two neighbor lasers. As a result, the lines on the edge of the scan may receive less thermal energy, causing less transfer of colorant.
Ruling and screen angle can also contribute to the artifacts. In halftone printing, printed images are formed from halftone dots, with the halftone dots varying in size according to the lightness or darkness of the image. The halftone dots are printed by the lasers, but the halftone dots are generally much bigger than the laser beams. Each laser prints in units of xe2x80x9cpixels,xe2x80x9d and usually a matrix of several pixels is required to make up a single halftone dot. Consequently, it may take several lasers in the array to print a single halftone dot. Halftone dots are printed at a defined ruling, i.e., a number of halftone dots per unit of length, and at a defined screen angle, i.e., an angle at which the rows of halftone dots are oriented.
In standard four-color printing, each color is printed using approximately the same ruling, but each color is xe2x80x9cscreenedxe2x80x9d at a different angle to prevent halftone dots of different colors from printing one on top of the other and obscuring each other. Screening colors at different angles also avoids generation of an objectionable moirxc3xa9 pattern between the color layers.
When the spatial frequency of the swath lines and the spatial frequency of the halftone dots are close to one another but not equal, a moirxc3xa9 pattern may form. In other words, two small superimposed patterns of similar spatial frequency may combine visually to form a larger and more prominent moirxc3xa9 pattern. The moirxc3xa9 pattern usually resembles spurious light and/or dark bands in the image, and is generally considered to be an undesirable aberration of the image. The phenomenon may also be described as xe2x80x9cbeating,xe2x80x9d because it results from superposition of patterns close to each other in spatial frequency. In a multicolor image, a plurality of screen angles come into play, and beating may occur in one or more colors, resulting in superimposed moirxc3xa9 patterns.
Stochastic printing is an alternative to halftone printing, in which color density is related to the spatial density of pixels printed by lasers. Moirxc3xa9 patterns tend not to form with stochastic printing because stochastic printing does not employ rulings and screen angles. In stochastic printing, however, swath lines may generate undesirable banding artifacts.
To reduce the artifacts described above, techniques may be employed to break up and/or reduce the swath lines, thereby reducing banding. In general, the techniques provide for overlapping swaths and providing masking for one or both passes that print the overlapped region. The masks superficially resemble randomized patterns of logical values, but for best performance, the masks are not fully randomized.
Masking, while reducing some banding artifacts, may introduce new banding artifacts. In particular, each laser in the array, when printing, derives some benefit from the heat generated by its neighbor lasers. An end laser, which has only a single adjacent neighbor laser, derives less thermal benefit than other lasers and consequently may print lower density pixels. As a result, the line width of an end laser may be narrower than the line width of other lasers, which can lead to banding. Another potential source of banding is too much transfer of colorant caused by overlapping swaths. The banding artifacts are especially evident along the edges of the swath.
The invention presents techniques for enhancing the masking to reduce or eliminate the new banding artifacts. The techniques generally provide for a special masking of one or more selected lines on the first swath, and for masked printing on the second swath.
In one embodiment, the invention presents a method for printing with a laser imaging system having a plurality of lasers. The method comprises printing a first line on a thermally sensitive medium with a laser modulated as a function of a set of data, and thereafter printing a second line on the thermally sensitive medium with a laser modulated as a function of the set of data and a mask. The second line overlaps the first line.
Because the image data are printed in the first line, printing the second line does not fill in any missing pixels, but merely over-prints some of the previously printed pixels. The second line, therefore, does not add much to the printed image. Because the second line was printed in a masked form overlapping the first line, the narrowness of the second line is not noticeable and the risk of transfer of too much colorant is reduced. This technique provides a thermal benefit to lasers neighboring the laser that prints the second line.
In another embodiment, the invention presents a method, which comprises printing a first line on a thermally sensitive medium with a first laser modulated as a function of a first set of data and printing a first set of contiguous lines on a thermally sensitive medium with a first set of lasers modulated as a function of a second set of data and a first mask. The method also comprises printing a second line on the medium overlapping the first line with a second laser modulated as a function of the first set of data and a second mask, and printing a second set of contiguous lines on the medium overlapping the first set of lines with a second set of lasers modulated as a function of the second set of data. In this embodiment, some or all lines of the image data may be printed twice.
In a further embodiment, the invention presents a system, which includes a thermally sensitive color donor including colorant, a receptor positioned to receive colorant from the donor, a controller and a set of lasers. Each of the lasers emits a beam directed at the donor. The controller controls a first laser as a function of image data and a first mask, the controller controls a first subset of lasers as a function of the image data, and the controller controls a second subset of lasers as a function of the image data and a second mask.
The details of one or more embodiments of the present invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the present invention will be apparent from the description and drawings, and from the claims.